Messages in this thread | | | Date | Mon, 26 Apr 2021 19:13:33 +0200 | From | Peter Zijlstra <> | Subject | Re: static_branch/jump_label vs branch merging |
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On Thu, Apr 22, 2021 at 01:31:51PM -0500, Segher Boessenkool wrote: > It sounds like you want it to behave like attribute((pure)) already is > documented as doing. Please open a PR? https://gcc.gnu.org/bugs.html > (We need buildable stand-alone example code, with what flags to use, and > something like what should happen and what did happen).
Something like so?
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gcc (Debian 10.2.1-6) 10.2.1 20210110
Clear fail on both counts, the first emits _3_ nops, where 1 is expected, and the second emits the nop _inside_ the loop.
0000000000001180 <elide_branches>: 1180: 48 83 ec 08 sub $0x8,%rsp 1184: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1) 1189: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1) 118e: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1) 1193: 48 83 c4 08 add $0x8,%rsp 1197: c3 retq 1198: 0f 1f 84 00 00 00 00 00 nopl 0x0(%rax,%rax,1) 11a0: 48 8d 3d 5d 0e 00 00 lea 0xe5d(%rip),%rdi # 2004 <_IO_stdin_used+0x4> 11a7: 31 c0 xor %eax,%eax 11a9: e8 a2 fe ff ff callq 1050 <printf@plt> 11ae: eb d9 jmp 1189 <elide_branches+0x9> 11b0: 48 8d 3d 5a 0e 00 00 lea 0xe5a(%rip),%rdi # 2011 <_IO_stdin_used+0x11> 11b7: 48 83 c4 08 add $0x8,%rsp 11bb: e9 80 fe ff ff jmpq 1040 <puts@plt> 11c0: 48 8d 3d 45 0e 00 00 lea 0xe45(%rip),%rdi # 200c <_IO_stdin_used+0xc> 11c7: 31 c0 xor %eax,%eax 11c9: e8 82 fe ff ff callq 1050 <printf@plt> 11ce: eb be jmp 118e <elide_branches+0xe>
00000000000011d0 <hoist>: 11d0: 53 push %rbx 11d1: 31 db xor %ebx,%ebx 11d3: eb 16 jmp 11eb <hoist+0x1b> 11d5: 0f 1f 00 nopl (%rax) 11d8: 89 de mov %ebx,%esi 11da: 48 8d 3d 36 0e 00 00 lea 0xe36(%rip),%rdi # 2017 <_IO_stdin_used+0x17> 11e1: 31 c0 xor %eax,%eax 11e3: 83 c3 01 add $0x1,%ebx 11e6: e8 65 fe ff ff callq 1050 <printf@plt> 11eb: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1) 11f0: b8 0a 00 00 00 mov $0xa,%eax 11f5: 39 c3 cmp %eax,%ebx 11f7: 7c df jl 11d8 <hoist+0x8> 11f9: bf 0a 00 00 00 mov $0xa,%edi 11fe: 5b pop %rbx 11ff: e9 2c fe ff ff jmpq 1030 <putchar@plt> 1204: 0f 1f 40 00 nopl 0x0(%rax) 1208: b8 05 00 00 00 mov $0x5,%eax 120d: eb e6 jmp 11f5 <hoist+0x25> 120f: 90 nop
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/* gcc -O2 -o pure-fail pure-fail.c */
#include <stdio.h> #include <stdbool.h>
#define __pure __attribute__((__pure__))
#define __stringify_1(x...) #x #define __stringify(x...) __stringify_1(x)
#ifndef __x86_64__ #define BYTES_NOP4 0x8d,0x74,0x26,0x00 #define BYTES_NOP5 0x3e,BYTES_NOP4 #else #define BYTES_NOP5 0x0f,0x1f,0x44,0x00,0x00 #endif
# define __ASM_FORM(x) " " __stringify(x) " " # define __ASM_FORM_RAW(x) __stringify(x) # define __ASM_FORM_COMMA(x) " " __stringify(x) ","
#ifndef __x86_64__ /* 32 bit */ # define __ASM_SEL(a,b) __ASM_FORM(a) # define __ASM_SEL_RAW(a,b) __ASM_FORM_RAW(a) #else /* 64 bit */ # define __ASM_SEL(a,b) __ASM_FORM(b) # define __ASM_SEL_RAW(a,b) __ASM_FORM_RAW(b) #endif
#define __ASM_SIZE(inst, ...) __ASM_SEL(inst##l##__VA_ARGS__, \ inst##q##__VA_ARGS__) #define __ASM_REG(reg) __ASM_SEL_RAW(e##reg, r##reg)
#define _ASM_PTR __ASM_SEL(.long, .quad) #define _ASM_ALIGN __ASM_SEL(.balign 4, .balign 8)
#define asm_volatile_goto(x...) do { asm goto(x); asm (""); } while (0)
/* --- */
struct static_key { int enabled; };
#define STATIC_KEY_INIT_TRUE { .enabled = 1, } #define STATIC_KEY_INIT_FALSE { .enabled = 0, }
static __always_inline __pure bool arch_static_branch(struct static_key * const key, const bool branch) { asm_volatile_goto("1:" ".byte " __stringify(BYTES_NOP5) "\n\t" ".pushsection __jump_table, \"aw\" \n\t" _ASM_ALIGN "\n\t" ".long 1b - ., %l[l_yes] - . \n\t" _ASM_PTR "%c0 + %c1 - .\n\t" ".popsection \n\t" : : "i" (key), "i" (branch) : : l_yes);
return false; l_yes: return true; }
static __always_inline __pure bool static_key_false(struct static_key * const key) { return arch_static_branch(key, false); }
static __always_inline __pure bool static_key_true(struct static_key * const key) { return !arch_static_branch(key, true); }
/* --- */
static struct static_key key_A = STATIC_KEY_INIT_FALSE;
/* * Expect: * * if (static_key_false(&key_A)) { * printf("ponies "); * printf("are "); * printf("small\n"); * } */ void elide_branches(void) { if (static_key_false(&key_A)) printf("ponies "); if (static_key_false(&key_A)) printf("are "); if (static_key_false(&key_A)) printf("small\n"); }
/* --- */
static struct static_key key_B = STATIC_KEY_INIT_TRUE;
static __pure int count(void) { if (static_key_true(&key_B)) return 10; return 5; }
/* * Expect: * * tmp = count(); * for (i = 0; i < tmp; i++) * printf("%d, ", i); * printf("\n"); */ void hoist(void) { int i;
for (i = 0; i < count(); i++) printf("%d, ", i); printf("\n"); }
/* --- */
int main(int argc, char **argv) { elide_branches(); hoist();
return 0; }
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